Bidirectional transistor amplifier



NOV. 17, 1953 BARNEY 2,659,774

BIDIRECTIONAL. TRANSISTOR AMPLIFIER I Filed Jim 7, 1949 o l l u. A

INVENTOR H. L. BA RNEY A T TORNE Y Patented Nov. 17, 1953 2,650,774 nmmiic'rromn TRANSISTOR AM Harold L. Barney, Madison, N. J., assignor to I Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June "I, 1949, Serial No. 97,677

This invention relates to signal translation networks utilizing semiconductor amplifiers as active elements.

A principal object of the invention is to provide substantial power amplification in each of two opposite directions of transmission.

A related object is to enable two-way commu-- nication to be carried out by way ofa two-wire line and associated unattended repeater equipment, without resort to switching apparatus.

Another object of the invention is to reduce the disturbing effect of current measuring equipment on a network in which a current is to be measured.

Another object of the invention is to supply a load with a voltage which, while it is dependent on aninput signal voltage, is independent of the impedance of the load. 1

Application Serial No. 11,165 of John Bardeen and W. H. Brattain, filed February 26, 1948, now abandoned, describes and claims an amplifier unit of novel construction, comprising a small block of semiconductor material, such as'N-type germanium, with which are associated three electrodes. One of these, known as the base electrode, makes low resistance contact with a face of the block. It may be a plated metal film. The others, termed emitter and collector, respectively, preferably make rectifier contact with the block. They may, in fact, be point contacts. The emitter is biased to conduct in the forward direction and the collector is biased to conduct in the reverse direction. Forward" and "reverse are here used in the sense in which'they are understood in the rectifier art. When a signal source is connected between the emitter and the base and a load is connected in the collector circuit, it is found that an amplified replica of the voltage of the signal source appears across the load. The aforementioned application contains detailed directions for the fabrication of the device.

The device may take various forms, all of which have properties which are generally simi- 6 Claims. (01. 179-170) Shive, Serial No. 44,241, filed August 14, 1948,

The device in all of its forms has received the appellation transistor, and will be so designated in the present specification.

In the Bardeen-Brattain application above referred to there is a tabulation of the performance characteristics of three sample transistors. one of these, it appears that increments of signal current which fiow in the circuit of the collector electrode as a result of the signal current increments which fiow in the circuit of the emitter electrode, exceed the latter in magnitude. This current amplification feature of transistors has become the general rule, and appears in nearly all transistors fabricated. It is discussed in detail in United States Patent 2,524,035, which issued October' 3, 1950, on an application of John Bardeen and W. H. Brattain, Serial No. 33,466, filed June 1'7, 1948, which is a continuationin part of the earlier application of the same inventors, which earlier application has now been abandoned. This feature is of such importance in connection with the present invention, as well as others, that the ratio of these increments has been given a name, In the present invention, the presence of such a current gain factor, not heretofore available in conventional vacuum tube amplifiers, is turned to account in the construction of a translation network having various new and useful properties, a principal one among these being that it is capable of providing substantial power amplification in either or both of two opposite directions.

The analogy of the transistor amplifier in its original form, that is with the base electrode common to the input and output-circuits, the input signal being applied between the emitter and the base and the output being taken from the collector and the base, to a vacuum tube amplifier circuit of the so-called grounded grid configuration has already been noted. Similarly, an analogy may be drawn between the conventional grounded cathode vacuum tube amplifier circuit and a transistor amplifier with grounded emitter. In the same way the grounded collector transistor amplifier is analogous in its operation to the grounded anode or cathode follower vacuum tube circuit. These three circuit configurations have long been accepted .as the Only ones in which a vacuum tube amplifier may taken from the base.

- 3 be satisfactorily operated to power gain simultaneously.

In an application of B. McMillan, Serial No. 96,485, filed June 1949, there is described a transistor amplifier circuit of a new configuragive voltage and 4 tion having certain novel features and advantages. -In this circuit the collector is common to the input and output circuits, the input signal being applied to the emitter and the output being It is, in effect, an inverted grounded collector transistor amplifier. An application of R. M. Ryder, Serial No. 96,500,

. filed June 1, -l949, describes certain new results which are obtainable with the circuit configuration of the McMillan application when the values of the associated network elements are appropriately selected. In particular, the Ryder application is based upon the discovery that, with appropriate parametric values for the inverted grounded collector circuit equal amplification may be obtained simultaneously in each of two opposite directions.

Y Another application of H. L. Barney, Serial No. 97,886, filed June 7, 1949, describes another transistor amplifier configuration in which the emitter electrode is common to the input and transmits them at a higher power level to the other end, and this without resort to any switching apparatus, either of the signal-controlled or the voice-controlled variety. Amplifiers adjusted in thismanner can be employed as. repeaters in such a transmission line and cascaded in any desired numbers.

The invention together with various other,"

features and advantages which it offers will be fully apprehended from the following detailed description of certain illustrative embodiments taken in connection with the appended drawings, in which:

Fig. 1 is a schematic circuit diagramof a.

inverted grounded base transistor amplifiernetwork which is adjusted to have anoutput impedance. which is substantially zero, and serves as a constant voltage source;

Fig. 2 is an equivalent network diagram of an inverted grounded basetransistor amplifier;

Fig. 3 is a schematic circuit diagram showing an inverted grounded base transistor amplifier output circuits, the signal being applied to the one in which the base electrode is common to v the input and output circuits, the signal being applied in a novel manner to the collector electrode, and the output being taken .from the emitter electrode. Withsuitable values of the associated impedance elements,- this circuit has 'certain striking new characteristics oilering marked advantages. First, with appropriate values of the associated impedance elements, the input impedance of the circuit may be made to have a substantially zero value. It is well known that current measuring equipment should have the lowest possible value of input impedance, in order that the introduction of such equipment into a circuit in which flows a. current to be measured shall have the least possible disturbing effect on the networl-r.v The circuit configuration of the invention is therefore well suited for use'in current measuring equipment.

fFor another thing, by adjustment of associated impedance elements to different values, the output impedance of the circuit of the invention may be made to have a substantially zero value. With'these adjustments the new circuit is suitable for use as a constant voltage source; that is, a network which, when a load is connected to its output terminals and'a signal source to its input terminals, delivers an output voltage to the load, which, while it is dependent on the magnitude of the signal source, is independent of the impedance of the load.

a By still other adjustments of the values .of the associated impedance elements, the amplifier circuit of the invention may be made to furnish power amplification simultaneously in each of two opposite directions. An amplifier of this sort, evidently, may be used as a repeater in a twoway, two-wire transmission system. It amplifies network which is adjusted to have a zero input impedance, employed as a current measuring device; and,

Fig. 4.is a schematic circuit diagram of a twoway, two-wire transmission system employing a plurality of inverted grounded basetransistor amplifiers coupled together in cascade.

Referring now to the drawings, Fig. 1 shows a transistor amplifier of the grounded base configuration. The transistor itself comprises a block I of semiconductor'material such as garmanium having a low resistance base electrode I in contact with one face thereof and two point contact electrodas in closely spaced contact engaging the opposite face. The contact point 3 is the emitter contact and the nearby contact 4 is the collector contact. As fully described in the aforementioned applications of John Bardeen and W. H. Brattain, the transistor operates best in the conventional manner when the emitter electrode 3 is biased positively with respect to v input transformer I 0 whose primary winding is the base by a fraction of a volt while the collector 4 is biased negatively by 40 to 100 volts. In the figure a battery I supplies the large negative bias to the collector while the emitter bias is supplied as the difference between the voltage drop across two resistors O, I, which are connected to the base 2 and another resistor l which is connected to the emitter I. The resistor I may be shunted, for signal frequency purposes, by a-condenser I, in the manner described in an application of H. L. Barney, Serial No. 49,951, filed September 18, 1948, and thereafter abandoned in favor of a continuation-impart application of H. L. Barney, Serial No. 123,507, filed October 25, 1949. now Patent No. 2,647,958, issued August 4, 1953.

In accordance with the invention, however, the standard practice for a grounded base network is departed from by applying the input signal not to the emitter electrode but to the collector electrode. This may be done by any convenient means, for example by the interposition of an connected to the terminals of a signal source ll while its secondary winding is connected, by way of the collector bias battery I, to the collector electrode 4 and to ground.

A load l2 requiring constant. voltage is connected tothe output terminals which, in accordance with the invention are the base (or ground) and the emitter. In the usual case it is prefersignals received from either end of the ine and able that steady biasing current be excluded sistor are represented by a fictitious internal generator of voltage v e'=rn.i (1) where K is is the emitter current, and

Tm is the mutual resistance of the transistor.

As the emitter current is is simply the second mesh current i: of Fig. 2, the fictitious internal generator voltage may be expressed as e'=rmiz (2) A terminating resistor El and an external source of electromotive force e1. are connected to the left-hand terminals of the equivalent network,

while a terminating resistor R1. and another source of electromotive force e2 are connected-to the right-hand terminals of the network.

In the light of the equivalent network of Fig. 2, the operation of the constant voltage network 01 Fig. l, and the manner in which the values of the various impedance elements are to be adjusted to obtain optimum preformance can be explained as follows:

The requirement that the voltage across the load shall be constant, that is, that it shall be independent of the impedance of the load itself and of the current through it, is equivalent to a requirement that the impedance of the transistor network looking into its output terminals shall have the characteristic of ashortcircuit, or zero resistance. That an adjustment of the values of the parameters of the network producing this result is possible may be seen in a qualitative sense from the following:

Because of the coupling between the two meshes of Fig. 2, the current is flowing in the right-hand mesh is dependent on the magnitude of the left-hand mesh current i1, which in turn is dependent on the magnitude of the fictitious generator voltage e'=r1'niz Assume for the moment that Rb is infinite R1. andei are both zero. Then,

and the voltage drops around the circuit. looking into the output from the terminals to which e: is connected, are equal to these voltage drops add up to zero, and thus the impedance seen from the output terminals is in eifect a short circuit or zero impedance.

and

(3) But if In the practical case when R1. and Rs are-both ance Tm is sumciently large, a small change in e: tending to increase i2 will finally result in a change of 2 such that the total increased voltage drop in the circuit and load R1,, required to bal ance the new value of ezLwill be absorbed in the load Rn. This is equivalent to stating that the output circuit to which Rx. and the source ea are connected, has zero impedance.

The precise conditions under which this result is obtained appear as consequences of the following computations:

Referring to Fig. 2, the mesh current equations are base.

It imposes no restriction on the computation of the output impedance to put RL=0 and e1=0. Making. these simplifications and solving Equations 4 and 5 for i: in terms of ca gives, for the output impedance,

Evidently, from Equation 7, Zout becomes zero and when the second term is equal to -Te; i. e., when Thus, for Zout=0, Tm nmst be larger than re,

and therefore a, which is defined approximately must be greater than unity. Ii, in addition, a exceeds unity by such a margin that R11: i 'bRl.

( .-+r=+rb) L+ a+ b) b( 'b+ n) If, however, Tm has the value given by (8) above, then Equation 12 reduces to 2 L i L+ c L+ b L i F. c+ b which is seen to be independent of R1. but linearly dependent on e1. It is also to be noted that if 1'' is made suiflciently large, the value of output voltage izRr. approaches the value of e1. With diflerent values of the circuit elements.

the inverted grounded base transistor amplifierpresents an input impedance which is essentially zero in magnitude.

' Referring again to the equivalent network 01' Rb is the external resistor in series with the l I'ig,2,assumefirstthatmisinflnite andthat Rais zero; that is, the output is short circuited.

assume Ri= so that the input electrometive force is applied directly between the collector and the base.

The sum of the voltage drops in the resulting single mesh is equal to the input voltage e1. These voltage drops comprise a positive voltage drop across 1'. and re,-which is equal to (1 'e+Te)i1 and a negative voltage drop across the fictitious internal generator equal to nuii. It Tm is sumeiently large so that these voltage drops add up to zero, the input impedance of the network is zero. Thus through the emitter, i: is thus reduced by the diversion of some of the current through the base, the magnitude of the fictitious generator voltage e is correspondingly reduced. In order that the negative voltage drop may then still be equal to the positive voltage drfip in the input mesh, it is necessary for the value of Tm to be still larger than-that given by (15). This in turn requires the value or a to be appreciably greater than unity. "With the assumption that e==0 and R|=0, and letting. R1. assume a finite value, the Equations 4 and may be solved for the input impedance 2:11, which is equal to This inputimpedance is zero when R1. is ad- Justed to make 4 rtira e) Despite the fact that the input impedance of the network is zero, power flows into the load.

The amount of this power may be calculated by setting R1 and e: both equal to zero in Equations 4 and 5 and solving for i: in terms or 11. This Elves ELT-W; 8

' irom which the output power is would have without this resistor. Thus, with a ypical transistor whose constants are But, it an external resistor Rs of 1,400 ohms be connected in series with the emitter, then,

r's rt-i-R't=2000 ohms Now R1. is calculated from Equation 1'1 to be 1,320 ohms, and the output power becomes, from Equation 19, 361k". For larger values of resistance added in the base electrode lead, the output power is increased, reaching a value of (Tar-Tc-Te) 11 at the point where Rb is infinite.

The circuit of Fig. 3 illustrates an application of the inverted grounded base amplifier to the measurement of current in the resonant circuit of an oscillator, here exemplified by a groundedbase transistor oscillator network of the type which forms the subject-matter of an application of H. L. Barney, Serial No. 67,159, filed December 24, 1948, which oscillates at the frequency to which the tank circuit comprising a coil 11 and a condenser I! are .tuned. Batteries Ee and Ee furnish bias potentials to the emitter and the collector, while the transistor is protected from injury by inclusion of a series resistor Rs, which may be by-passed by a condenser Cs.v In thisexample, it is considered undesirable to introduce any appreciable impedance into the resonant circuit, by insertion oi. the measuring circuit. The measuring circuit, comprising an inverted grounded base transistor amplifier with output load consisting of a meter I1 shunted by a resistor 8 has essentially zero input impedance by virtue of adjustments of the load impedance as described above.- The input to the measuring circuit is taken through a transformer 10 in order to prevent the flow of biasing current, ascalled for by the measuring transistor, through the tuned circuit of the oscillating transistor. The transformer is preferably one having a minimum of leakage reactance and a high coupling coeiilcient between windings in order to minimize its eiiect when inserted between the resonant circuit and .the input to transistor. Operation of switch I! to the left leaves the oscillation generator in the normal condition. When the switch la is operated to the right, the input terminals of the current measuring circuit are connected in series with the coil of the resonant circuit, in which condition the deflection of a meter, when suitably calibrated, indicates the magnitude of the current in the resonant circuit. Alternatively a cathode ray oscilloscope or other indicating means may be employed in place of the meter to indicate the wave shape of the current flowing in the resonant circuit, Other applications of the measuring circuit of Fig. 3, in cases where zero or very low input impedance is required of the measuring circuit in order to avoid disturbing the operation of the circuit to be measured, may suggest themselves to those skilled in the art.

the quantities specified inEquations 21 and 24 are both greater than unity. Since the insertion gain from left to right is smaller than from tial power gain. It is shown below that with certain specified adjustments of circuit parameters; the grounded base circuit may be used to transmit simultaneously in both directions with substantial gains. I

The insertion gains of the amplifier in the two directions may be calculated using Equations 4 and 5. Solving first for the current in the second mesh of Fig. 2 for a given value of c; with e:=0, gives .If the amplifier stage were not in circuit betweenRi and Rr., the current would have been The ratio of the current after insertion of the Ill amplifier, to the current before insertion, which is here referred to as insertion gain, is then Insertion gain (left to right) i i :)m u. 2 a+ 'b)( L+ -i-rb) r ,(r +r,,,) For the opposite direction of transmission, right to left in Fig. 2', the current i1 is calculated from (4) and (5) with e1=0, giving Insertion gain left-right r' Insertion gain, right-left r' +r,,,

In conventional transistors, Tb is much smaller than Tm, so that without the addition of an external resistor Re, the term (r'b-l-rm) would have a much larger absolute value than 1'', indicating a much larger gain in the right-left directionof transmission. When, however, the effective magnitude of Tb is increased by the inclusion of a resistance Rb in series with Tb, the gains in the two directions become greater, and also more nearly equal.

Both left-to-right and rightto-left insertion gains of the amplifier stage will be positive when right to left, it may be stated that the amplifier will have positive gains simultaneously in both directions if b( i+ L) (R.-+u+rr).(Rt-l-r.+r's)ri(ri (2 This may be simplified to the condition that This expression is indicative of several attributes of the inverted grounded base amplifier. In the first place, it shows that r mustbe' i'el-ter than Tc, and thus a must be'larger than unity. Secondly, it shows that an increase in 1'' (which may be obtained by increasing Rh) redueemthe magnitude of the expression to the right of'the inequality sign, and thus tends to assure-that a condition will be set up in which positive gain will result. Furthermore, it shows that increasing the value of R1 and Rh does just the opposite. and if they. are sufilciently increased, the inequality will no longer exist, and positive gain will not be exhibited by the amplifier.

For stable operation of the transistor amplifier, another criterion must be met, as taught in an application of H. L. Barney, Serial No. 08,684.

filed November 6, 1948, which first issued as Patent No. 2,585,077 on February 12, 1952, thereafter surrendered in favor of Reissue Patent'No. 23,563, issued October14, 1952, which criterion is stated as follows:

when the resistances external to the transistor are all considered to be zero. If the external resistances of Fig. 2 are included, the Expression 29 may be rewritten as The two Expressions 28 and 30 then define the conditions under which stable positive gain may be obtained with the inverted groundedbase transistor amplifier, and they may be combined as follows:

which may be rewritten, to express the restrictions on T'b, as follows:

stants may be assumed for a transistor and samciated circuit elements as follows:

-re=500 ohms -7b=600 ohms Tc=20,000 ohms rm=40,000 ohms R1=5,000 ohms RL=5,000 ohms Rb=12,400 ohms 11 12 The insertion gain in decibels from i left to L: are presented with similar impedances in the rightisgiven, from Equation 21 by sense that if, at either end of the line the imb-l-Rt)( i+Rs) I I (Ri+ t+ 'b+Rb)( L+ b+ Uf( b+Rb)( b+ B+ ,(33)

' Substituting the above assumed values gives 'pedance looking into the line is that of a trailsistor collector, so too is the impedance looking Gam=19Ade1be1s into the other end of the line. Similarly, 11 the The insertion gain in decibels, from right to stages are so connected that one of the-terminatleft is given by Equation 24 as mg networks N1, N2 sees an emitter impedance, so

J b+ b+ n)( i+ L) 34 9? (RH-n+1't-I-Rt) (R1.+ e+rt+Rt)"'( b'F UUs-I-Rrt -i) Substituting the above assumed valuesgives too does the other terminating network. If, on

.- the other hand, an odd number of stages is em- Gain=315 decibels ployed and alternate ones are inverted, then the 4 l .loads necessarily see difierent line The assumed values may be shown to satisfy sauces and the criterion for stability,. as stated in Expresimpedance-s and impedance ng devices 1 30, n such as transformers are advantageously em- Thus, for small'signal voltages which do. not 'Plqyed to Prevent 9?? due impedance exceed the linear range of the transistor charmismatch- .acteristics, transmission of signals may proceed s M immrmnthoweverthan the consider in both directions simultaneously without inter- 9451011 of symmetry as between each terminating modulation or other interference network and the line of cascaded amplifiers, is

I 4 illustrates the application of such a mthe consideration that the output impedance of .lateral amplifier in a two-wire transmission sysany 8 of the Sequence automatically matches .tem such as a long tollcil-cuit for speech. The the input impedance of the stage to which it is .-properties of the bilateral amplifier stage just coupled .described are used to compensate for loses in the A vlrtue of this em t is that by the intervening sections of line between the several P 1 selection of l tum ratios in stages. To such stages, 2|, 22, are shown in Big. Well-FROM ma ne it an be a anged that the d coupled to the line by transformers at input eInItter-to-base terminals of each transistor amand output. At each end of the line are conp fi Whether t y be regarded as pu t l nected hybrid coil terminating sets, with the mlnals or as output terminals, can be made to see sources S1, S2 connected to terminals 23, 24 and impedance. o g into the line which interthe receivers or loads L1, L2 connected to terconnects two successive stages, of the proper value minals 25, 26. The networks N1, N: are adjusted to enable he W lacent transistor ampll-" to balance the impedance of the line connected r o furnish qual gains in their inverted or to the hybrid coils, so as to suppress direct transin their normal transmission directions. 'The mission from each source to the load at the 40 Same is true of the other pair of terminals of the same end of the line in the customary manner. amplifier, namely the collector-to-base termi- In transmission from left to right in Fig. 4, signals. Thus, referring to the foregoing example nals from the source Si divided in the left end of a grounded base amplifier which, with aphybrid coil, with half the energy going into the V propriate values of the circuit elements, gives a balancing termination N1, and the other half to gain of 31.6 decibels in the forward direction and the line. The signal isattenuated by the line im- 19.4 decibels in the reverse direction, it is a simpedances 21, 28, 29 andis amplified by the two ple matterrto select the turn ratios of the various transistor amplifier stages 2|, 22, to compensate transformers which couple the several amplifier for this attenuation. 0n reaching the other hystages to the intervening line sections in such a brid terminating set the energy again divides, way that the collector-to-base terminals of each half going to the load L1 and half into the source stage see an impedance of 5,000 ohms while at 82. Transmission of signals from the source S: -the same time-the emitter-to-base terminals of in the opposite direction, 1. e., from right. to left. each amplifier stage also see an impedance of In p ds n an logous manner to 5,000 ohms. These values of 5,000 ohms are the that just described, and as stated above may take values of R1 and R1. which enable the typical place simultaneously with transmission from left transistor whose internal parameters are given wright, in the above example tofurnish the specified. In the figure the left-handamplifler stage 2| gains simultaneously in t in t is shown as of the inverted type for transmission Va i th us d adaptations of th from left to right, while the right-ha d plifi r inverted grounded base amplifier of the inven- 'stage 22 is shown as the inverted type for transtion will occur to those skilled in the art.

mission from right to left. The roles of these What is claimed is: two amplifier stages are reversed for transmission 1. An amplifier of which the active element ininthe opposite direction, cludes a transistor comprising a semiconductive Although it is entirely possible to connect any. body having a base electrode, an emitter electrode,

number-or all of a group of cascaded grounded and a collector electrode'all in operative contact base transistor amplifiers either in the inverted therewith, a source connected to supply potencircuit configuration or in the conventional cirtials to said electrodes for transistor operation, cult configuration, it is preferred to invert alterinput terminals connected to the base and to nate members of the roup- In this manner, the the collector, respectively, output terminals co n-.

difference between left-to-right and right-to-left nected to the'base and to theemitter, respec gains does not add up over a number of stages. tively, and a load connected to said output ter-. but may be averaged out with an even number mmals, the resistance of said load 'being pro-.

of stages. Also, when, as in the example shown portiorled according to the formula there are two or any even number of amplifier R stages, the signal sources S1, S2 and the loads L1, f--

To is the emitter resistance of the transistor it is the collector resistance of the transistor rs is the base resistance of the transistor rmis the mutual resistance of the transistor Rb is the external resistor connected in series with the base.

where said input terminals, the resistance of said terminating resistor being proportioned according to the formula where 7's is the emitter resistance of the transistor rs is the base resistance of the transistor To is the collector resistance of the transistor Tm is the mutual resistance of the transistor Rb is an external resistor connected in series with the base,

said amplifier being characterized by an output impedance of substantially zero magnitude.

3. An amplifier of which the active element includes a transistor comprising a semiconductive body having a base electrode, an emitter electrode, and a collector electrode all in operative contact therewith, a source connected to supply potentials to said electrodes for transistor operation, input terminals connected to the base where r's rc-i-Rs Te is the emitter resistance of the transistor Th is the base resistance of the transistor To is the collector resistance of the transistor Tm is the mutual resistance of the transistor Rs is an external resistor connected in series with the base,

said amplifier being characterized by a load voltage which is dependent on the signal of the source but is independent of the resistance of the load.

ii. A bilateralamplifier fortransmitting s :51. with simultaneous, substantial power gains in a forward direction and in a reverse direction, said amplifier including a transistor comprising a semiconductive body, an emitter electrode, a collector electrode, and a base electrode all in operative contact with said body, a first signal input-output circuit including said base electr and said collector electrode for input signals in said forward direction and output signals in said reverse direction, a second input-output circuit including said base electrode and said emitter electrode for input signals in said reverse direction and output signals in said forward direction, said first and second signal input-output circuits having a common portion including said base electrode'wherein the external portion Rb of said common circuit has an impedance which is proportioned to satisfy the relation 5. In combination with a signal source and a load, a bidirectional transmission system which comprises a plurality of bidirectional transistor amplifier stages coupled together in cascade, each of said stages comprising a three-electrode transistor amplifier of the grounded-base configuration, the emitter electrodeof one stage, being coupled to the emitter electrode of the following stage, the collector electrodes of the first and last stages of the plurality being coupled to the source and to the load, respectively.

6. Apparatus as defined in claim 5, wherein the number of said stages is even, whereby the gain of said entire system has the same value for signals transmitted in each of twoopposite directions despite inequalities between the gain of any single stage for signals transmitted in one direction and the gain of said stage for signals transmitted in the opposite direction.

HAROLD L. BARNEY.

References cited in the file of this patent UNITED STATES PATENTS Number Great Britain Dec. 6, 1985 

